His Girl Friday is one of the most famous films in the public domain—but with an interesting loophole. The play on which the film is based, 1928’s The Front Page, was copyrighted and renewed, which means it’s still protected and therefore so is any work directly based on it. This makes His Girl Friday practically unusable in terms of free exhibition, because you’d need permission from the copyright owners of The Front Page, even though the film is, technically, public domain.
https://www.mentalfloss.com/article/92931/11-classic-films-public-domain